Ball bats play an integral role in amateur, school, college and professional ball sports, such as baseball and cricket. Bats are generally made of metal, composites, or various combinations thereof. Bats can be monolithic (i.e., single material) in the barrel region, or multiwall in design (i.e., multiple materials). The history of metal bats is significant and can be traced to the early 1970's when the aluminum bat was first developed and commercialized. The National Collegiate Athletic Association (NCAA) approved aluminum bats in 1974 and aluminum has been the dominant metal bat material for decades.
Bats have generally been improved to increase the rebound velocity of the ball struck with the bat. Unlike wood bats, hollow aluminum or composite bats have variable barrel stiffness, defined as the hoop frequency. Reducing the hoop frequency of a bat equates to softening the barrel, resulting in an increase in batted ball speed. This can be accomplished by decreasing the wall thickness of the hollow bat and/or decreasing the stiffness of the barrel region of the bat. Stronger, albeit more expensive, metal alloys allowed bat designers to “thin-out” the impact zone without the bat denting, resulting in more of a trampoline effect and higher and higher performance. The historical trend in metal bats has been to thin out the metal wall, decrease stiffness, reduce hoop frequency and increase rebound velocity.
Recent changes have been implemented by various administrative bodies, such as the NCAA, limiting rebound velocity of ball bats for player safety. A new “Batted-Ball-Coefficient-Of-Restitution” (BBCOR) requirement went into effect for college players Jan. 1, 2011 and for high school players Jan. 1, 2012. The BBCOR standard replaced the previous “Ball-Exit-Speed-Ratio” (BESR) requirement, which had been in place for many years. The BBCOR requirement is a lower rebound velocity requirement than BESR, and meeting the BBCOR requirement means moving back down the rebound velocity versus barrel stiffness curve in the direction of wood bats (i.e., stiffer bats having lower rebound velocity). The new BBCOR requirement has thus hampered 40 years of innovation in “hotter bats” that trended toward maximizing rebound velocity through better alloys and more creative designs that decreased barrel stiffness.
The challenge today is to find ways to increase bat performance without increasing the weight of the bat. Presently, meeting the BBCOR requirement by simply increasing the wall thickness of a high-strength aluminum alloy bat, results in increased barrel weight and cost. The increase in barrel weight is particularly problematic in that it makes it more difficult for players to swing the bat fast, (and thus hit the ball further), and it adversely effects balance of the bat, making them feel heavy. Given these challenges, there is an ongoing need to design improved high-performance bats that meet the new BBCOR requirements while minimizing barrel weight and cost.